1. Field of the Invention
This invention deals with the production of nickel base superalloy articles having an anisotropic elongated grain structure suited for use at elevated temperatures. A thermomechanical treatment is used on super-plastic materials in the solid state, followed by annealing using a thermal gradient.
2. Description of the Prior Art
The mechanical properties of metals are strongly affected by grain boundaries. At low temperatures, grain boundaries are generally stronger than the material within the grains, but at high temperatures the reverse is true. At elevated temperatures, creep is usually observed to occur much more rapidly in fine grain materials than coarse grain materials. For this reason, coarse grained materials are usually preferred for stressed applications at elevated temperature.
Improvements in creep properties of coarse grain materials may be obtained if the grains can be significantly elongated in the direction of stress. This elongated grain material has significantly fewer grain boundaries transverse to the stress axis and accordingly has improved high temperature properties in the direction of grain elongation. As used herein, the term "elongated grain" is intended to encompass single crystal material, which are those characterized by the absenece of internal grain boundaries.
Two general techniques exist for producing such maaterial. One method, known as directional solidification (D.S.) involves controlling the heat flow and other conditions during the solidification process to produce an elongated microstructure. This technique is discussed in U.S. Pat. No. 3,260,505 issued to VerSnyder in 1964 of common assignee with the present application. The process described in this prior patent produces elongated grains in a cast structure.
The second method involves controlled recrystallization after deformation. In its best known form, this type of process involves straining the material a small, but critical amount, to produce a particular dislocation density, and then heating to above the recrystallization temperature under conditions which encourage grain growth rather than grain nucleation. This process produces elongated grains in a wrought type of structure. The heating is usually performed in a moving thermal gradient and the recrystallized grains tend to grow along the axis of gradient motion. The earliest use of type of process appears to have been in the electric lamp industry for the preparation of tungsten filament material. British patent 174,714 (1922) describes the application of the process. A further description of this process is contained in the book "Tungsten" by C. J. Smithells, Chemical Publishing Co., especially pages 143 through 146. Apart from use in the preparation of electric lamp filaments this type of process has been widely used as a metallurgical technique for the production of single crystals. This aspect is reviewed in the book "The Art and Science of Growing Crystals" edited by J. J. Gilman, John Wiley Publ. Co., (1964) pages 415 through 479. A mathematical model of the process has been developed by Williamson and Smallman in Acta Metallurgica, Vol. 1, pages 487-491 (1953). Additional references which bear on this subject include "Crystal Growth in Metals" by G. R. Fonda in General Electric Review, Volume 25, May 1922, pages 305-315; and "Ueber die Umkristallisation von Elektrolyteisen" by G. Wasserman in Mitt. K. W. Inst. Eisenf. Dusseldorf, Volume 17, 1935, page 203.
Extensive use of this type of process has been made for the production of special magnetic materials, see for example U.S. Pat. No. 3,219,496, and an article by Dunn and Nonken in Metal Progress, December 1953, pages 71-75.
More recent references to this type of process include U.S. Pat. Nos. 3,850,702, 3,746,581 and 3,772,090.
U.S. Pat. No. 3,850,702 describes a process applicable to the gamma/gamma prime alloys in which the alloy is heat treated to produce an all gamma structure prior to straining. The straining step is performed at relatively low temperatures and reprecipitation of the gamma phase occurs during annealing. Both U.S. Pat. Nos. 3,746,581 and 3,772,090 are processes applicable chiefly to dispersion strengthened alloys. In U.S. Pat. No. 3,772,090 the strain is imparted at low temperatures, while in U.S. Pat. No. 3,746,581 the strain is imparted by hot extrusion under controlled conditions.
The commercial use of the strain anneal process has heretofore been limited to simple shapes of constant cross section since it has hertofore not been possible to uniformly strain articles of varying cross section. Hence the process has been restricted to shapes such as wires, rods, bars and strips. One possible exception to this statement is found in U.S. Pat. No. 3,772,090 which states that a turbine blade shape was recrystallized using a moving gradient process. No details of the deformation process are given and it is not clear how a uniform dislocation density could be generated in such a structure by the processes discussed in the patent. It seems possible that the shape was that of a constant cross section airfoil without the root portion which must be present for practical application.
Another reference which appears pertinent to the present invention is U.S. Pat. No. 3,519,503, assigned to the present assignee. This patent discloses an isothermal forging technique which is applied to superalloys which have been rendered temporarily super-plastic by an appropriate prior thermomechanical conditioning treatment.